Disease and/or trauma cause deterioration of natural joints of the human body. Replacement of natural joints with joint prostheses can distinctly enhance the quality of life of an individual affected by such joint conditions. Various joint prostheses are known. Among the more common joint prostheses are those that replace all or part of the natural knee and hip joints.
Components of joint prostheses must be implanted and secured within existing bone. In the case of hip arthroplasty, a surgical technique in which the hip joint is replaced by a prosthetic joint, a cavity is prepared in a proximal portion of the patient's femur to receive a femoral stem (a portion of a prosthetic hip joint). Other joint replacement surgical techniques require the formation of similar cavities within existing bone for the installation of various prosthesis components.
Once such a cavity is prepared, the prosthesis component may be secured within the cavity by a number of techniques. For example, the prosthesis component may be cemented within the cavity, or it can be installed through mechanical fixation by a friction fit or the use of a fixation device.
When cement is used to secure a prosthetic component in place, it is usually desirable to create an even cement mantle around the component. One way to ensure a uniform cement mantle is to center the prosthetic component within the target cavity with the assistance of a mechanical guide secured to the prosthetic component. Such guides are commonly referred to as "spacers" or "centralizers." Known spacers for hip stems, for example, are snugly fitted to the distal end of the hip stem prior to the stem being plunged into a reamed femoral canal filled with bone cement.
Although the known spacers can be generally successful in centralizing the stem within the femoral canal, the configuration of the spacers and the techniques for using them cause a variety of problems with respect to the integrity of the cement mantle and the bond between the cement mantle and the prosthetic components. For example, depending on the bone cement selected, even a short delay from the time the cement is prepared and deposited into the canal, to the time that the stem and spacer are inserted therein, can allow the cement to cure or harden enough so that a sub-optimal bond between the cement and the spacer occurs. Additionally, the spacer can divide the cement mantle into three or four sections beyond the distal tip of the stem in a way such that the separated sections do not bond together.
Furthermore, as a mated stem and spacer are inserted into a bone cement filled canal, the spacer tends to drag air, in the form of small bubbles, from the surface of the cement at the point of insertion all the way to the final resting point of the spacer. The bubbles, many of which become trapped in the cement as it hardens, create discontinuities at the interface between the spacer/stem and cement that can lead to debonding. Because the spacer is at the distal end of the stem, bubbles are at the distal end; and the distal end is where the highest cement/stem interface stresses are produced. Also, when the cement hardens, it is likely that the spacer will not bond with the cement. Thus, not only do known techniques create the precursor of bonding failure, they do so in the most vulnerable location.
Therefore, it is desirable to provide additional structures and techniques that can be easily implemented and that address the challenges of hip stem fixation, and more particularly the problems associated with creating a robust cemented fixation.